668 REPOET— 1891. 



curvatures is so strictly identical as Frank supposed ? In the first place, it is not 

 clear why there should be identity of mechanism in the movements of organs or 

 plants of completely diflerent types of structure. The upholders of the identity 

 chiefly confine themselves to asseveration that a common explanation must apply 

 to both cases. I believe that light may be thivwn on the matter by considering 

 turgescence, not in relation to growth, but in regard to stability of structure. 



An acellular organ, such as the stalk of the sporangium of !Mucor, owes its 

 strength and stiffness to the tension between the cell contents and the elastic cell- 

 wall, but it does not follow from this that in multicellular organs strength and stiff- 

 ness are due to the sum of the strengths of its individual ceils. Indeed, we know 

 that it is not so ; the strength of a multicellular organ depends on the tension between 

 pith and cortex. It is, in fact, a model of the single cell; the pith represents the 

 cell-sap, the cortex the cell-wall. Here, then, it is clear that the function performed 

 by the cell-wall in one case is carried out by cortical tissues in the other. If this 

 is the case for one function there is no reason why it should not/ hold good ia 

 another, viz., the machinery of movement. 



If we hold this view that the cortex in one case is analogous with a simple 

 membrane in the other, we shall not translate the unity of acellular and multicellular 

 organs so strictly as did Frank. Indeed, we may fairly consider it harmonious with 

 our knowledge in other departments to find similar functions performed by morpho- 

 logically different parts. The cortex of a geotropic shoot would thus be analogous 

 with the membrane of a geotropic cell in regard to movement, just as we know that 

 these parts are analogous in regard to stability. 



In spite of the difficulties sketched above, one writer of the first rank, namely, 

 H. de Vries, has upheld the view that growth curvatures in multicellular organs ' 

 are due to increased cell-pressure on the convex side ; the rise in hydrostatic pres- 

 sure being put down to increase of osmotic substances in the cell-sap of the tissues 

 in question. Such a theory flowed naturally from De Vries' interesting plasmolytic 

 work.'- He had shown that tliose sections of a turgescent shoot which were in 

 most rapid growth show the greatest amount of shortening when turgescence is 

 removed by plasmolysis. This was supposed to show that growth is proportional 

 to the stretching or elongation of the cell-walls hj turgor. Growth, according to 

 this view, consists of two processes: (1) of a temporary elongation due to turges- 

 cence, and (2) of a fixing process by which the elongation is rendered permanent. 

 De Vries assumed that where the elongation occurred, its amount must be propor- 

 tional to the osmotic acti^^ty of the cell contents ; thus neglecting the other factor 

 in the problem, namely, the variability in the resistance of the membranes. He 

 applied the plasmolytic method to growth curvatures, and made the same deductions, 

 lie found that a curved organ shows a flatter curve' after being plasmolysed. This, 

 according to his previous argument, shows that the cell-sap on the convex is more 

 powerfully osmotic than that on the concave side. This again leads to increased cell- 

 Gtretching, and finally to increased growth. 



The most serious objection to De Vries' views is that the convex half of a 

 curving organ does 7jot contain a greater amount of osmotically active substance.* 

 It must, however, be noted in the heliotropic and geotropic curvature of pulvini, 

 there is an osmotic difference between the two halves '' — so that, if the argument 

 from uniformity is used against De Vries (in the matter of acellular and multi- 

 cellular organs), it may fairly be used in his favour as regards the comparison of 

 curvatures produced with and without pulvini. 



It is not easy to determine the extent to which De Vries' views on the mechanics 

 ■of growth curvature were accepted. The point, however, need not detain us, 

 for the current of conviction soon began to run in an opposite direction.* 



' Bot. Zeitung, 1879, p. 835. ' Ibid. 1877, p. 1. 



' Frank made similar experiments but failed to find any diminution of curvature. 



* Kraus, Abhand. Nat. Gesell. zu Halle, xv., 1882. See also a different proof by 

 Wortmann, Deutsch. Hot. Gesell. 1887, p. 459. 



^ Hilburg in Pfeffer's Tiihinf/en. Untermch., vol. i., 1881, p. 31. 



" An opportunity will occur later on for referring to some details of De Vries' 

 work not yet noticed. 



